I recently went to a bike mechanic and he dropped some kerosene on the cog and chain for lubing them. He said that it keeps the dirt off and is quite volatile so will evaporate after sometime leaving some part behind which acts as a good lube. He said that he knows this by experience. Is it good to use kerosene as lubricant?
No, Kerosene won't leave behind enough of a layer to protect the chain.
It might be useful as a solvent to clean the chain before applying a proper lubricant.
Kerosene is technically a paraffin which comes from the greek word wax. Kerosene is used as a fuel and a solvent. A waxy solid extracted from petroleum is called paraffin wax. Pouring a liquid kerosene on a chain is not going to leave much if any waxy lubricant behind.
Kerosene is not a good choice as a solvent as it is flammable.
It would only be effective if the chain was wet lubed, and clean. As a solvent Kerosene thin and redistributed the lubricant already on the chain. It will therefore have an effect at lubricating the chain as the pressure points will get lube migrating to them, provided small enough amounts were use that the lubricant was not washed off the chain.
Using kero this way would be better than not lubing the chain, however it would be better to use a lube and do it properly. Where I might use it is a chain that has been lubed then stored for a while to get things moving again - but in all cases it has no benefit over using a proper lube as a lubricant.
There is actually very little value in lubricating the chain, all you really need is to keep it clean and rust free. Kerosene seems about as good as anything for doing that. The primary reason to lubricate a chain is to keep water out of the rollers and just generally keep the chain rust free.
Most of the things people put on chains actually shorten the life of the gears by attracting dirt and dust. This grit combines with the grease to create a grinding paste that can wear gears in a relatively short time.
But how you take care of your bike chain is one of the great religious debates in the cycling world, everybody has a theory. For just about any grease/oil/wax out there you'll find somebody that swears by it as a bike chain lube.
Lube or don't lube, use the magic wonder fluid, all that really matters is that you check your chain for wear at regular intervals and replace it when it starts to show any signs of wear. Compared to all the other parts on your bike, chains are cheap, replace them often and your expensive cassettes and chainwheels will last a very long time.
Kerosene is not a good lubricant, but it's good for dissolving the greaser in a chain, and has long been used for cleaning chains. (Gasoline can also be used, but poses some safety issues!)
In the UK, kerosene is called paraffin, so British bike maitenance books recommend cleaning the chain in paraffin. In the US, "paraffin" means candle wax. I suspect the curious trend of washing a bicycle chain in hot wax comes from this transatlantic mistranslation.